In recent years, an optical communication network enabling large capacity data communication at high speed is expanding. The optical communication network is assumed to be mounted from intra-devices to inter-device in the future. A light guide that can be arrayed is expected to realize the print wiring substrate as an optical wiring.
The light guide has a double structure of a center core, which is called a core, and a capsule called a clad, where the index of refraction of the core is higher than the clad. In other words, the optical signal entered to the core is propagated by repeating total reflection inside the core.
In recent years, in particular, realization of a flexible optical wiring mounted on a smaller and thinner commercially-off-the-shelf device with the light guide is desired. A light guide having high bendability is being developed by using a material more flexible than the prior art for the material of the core and the clad of the light guide. The data transmission between the substrates in the device can be carried out with the light guide when using the light guide having high bendability.
A mechanism of light transmission in the light guide module using the light guide will be briefly described. First, a drive portion drives the light emission of the light emitting portion (optical element) based on an externally input electrical signal, and the light emitting portion irradiates the light incident surface of the light guide with light. The light applied to the light incident surface of the light guide is introduced into the light guide, and exit from the light exit surface of the light guide. The light exit from the light exit surface of the light guide is received by a light receiving portion (optical element) and converted to an electrical signal.
The resin sealing technique is generally applied in the package of the light transmission module. The optical element can be protected from humidity and dust by filling the sealing resin to the substrate, whereby the degradation of the optical element can be prevented (e.g., see Patent Document 1 and Patent Document 2).
However, the configurations of Patent Document 1 and Patent Document 2 have the following problems.
Specifically, in the configuration of Patent Document 1, the resin is difficult to fill without including air bubbles since the resin is filled to a gap between an optical device (14) and a film optical wiring (11) formed through a bump (10), and the light transmission module is difficult to produce while maintaining a stable quality.
As disclosed in Patent Document 1 and Patent Document 2, irregularities are form on the adhering surface of the resin and the light guide due to curing and contraction of resin if the sealing resin is filled so as to closely attach to the light guide.
Thus, variation occurs in the incident/exit direction of light between the light guide and the optical element due to air bubbles and curing and contraction, and the light coupling efficiency does not stabilize.
If the light coupling efficiency is not sufficiently maintained and the coupling loss becomes large, the probability the communication error occurs becomes high, and the light guide becomes in appropriate as a communication medium.
In order to solve such problem, a resin sealing technique for covering the optical element so that the sealing resin does not adhere to the light guide can be realized, as in the example shown in FIG. 2 and FIG. 3. FIGS. 2 and 3 are cross-sectional views of the light transmission module in a case where the package including the optical element is cut in a direction perpendicular to the light transmission direction in the light guide.
The light transmission module 90 includes a package 95, a cantilever member 94, a substrate 93, a light guide 92, and an optical element 91. The cantilever member 94 and the substrate 93 are mounted on the package 95 formed with a recess having the four sides surrounded by side walls raised from a bottom plate, and the optical element 91 is mounted on the substrate 93. The cantilever member 94 supports one end of the light guide 92. The sealing resin 96 is filled so as to cover the optical element 91, and forms a sealing surface 96a. 
As shown in FIG. 2 or FIG. 3, the sealing surface 96a of the sealing resin 96 does not adhere to the light guide 92, and thus the coupling loss due to curing and contraction can be reduced while protecting the optical element 91 with the sealing resin 96.